The problem is an out-of-range-problem in writing in the arrays.

for(int s = half; s < encryptArr.length; s++){
    second[s] = encryptArr[s];
  }

s is to high for second! s is only suitable for encryptArr.

Same here:

for(int j = 0; j < (encryptedText.length() / 2) + 1; j++){
      decrypted += second[j] + first[j];
    }

j at the kast step is too high for "first".

I said post your code and not your error message... ;-)

Maybe post your code here (mark as spoiler) and we will see... :-)